The invention relates to free piston internal combustion engines of the type comprising driving (or motor) cylinders provided with intake and exhaust ports and locating driving pistons arranged to reciprocate and provided with piston rings cooperating with said ports. The driving pistons are securely connected to compressor pistons located in compressor cylinders and cooperating therewith to deliver air under pressure.
The invention is particularly--although not exclusively--useful in free piston machines constituting gas generators supplying a gas flow at high temperature and pressure to a turbine.
One of the factors which limit the performance of such a machine and its service life between stops for maintenance is the temperature behavior of the components which define the combustion chambers in the motor cylinders.
In particular, the head piston ring of each driving piston (i.e. the ring closest to the combustion chamber) is subjected to very high thermal stresses. At the moment when the head ring initially uncovers the exhaust ports, a jet of burning gas escapes, sweeps the head ring, raises it to a high temperature and carbonises the lubricating oil which protects it against friction.
It may be mentioned that a somewhat similar problem exists in two-stroke Diesel engines, but it is less acute: hot gases tend to overheat the edge of the exhaust ports rather than the head rings. In a Diesel engine, that difficulty may consequently be overcome by circulating a cooling fluid around jackets in which the exhaust ports are formed. Such an approach is not feasible in a free piston engine.
It is an object of the invention to provide a free piston internal combustion engine in which the heating of the head ring of the driving pistons is substantially reduced; it is an other object of the invention is achieve that result without substantial added intricacy.
For that purpose, the invention has taken advantage of the fact that, in free piston engines, air is readily available at a moderate temperature and under a pressure greater than that which exists in the exhaust manifold.
According to the invention, each motor cylinder is provided with air delivery means for supplying air from the compressor cylinders as a jet directed along the driving piston and towards the head ring of the latter at the moment when the piston starts to uncover the exhaust ports.
The air jet at a much lower temperature than that of the exhaust fulfils several functions: it cools the piston rings, particularly the head ring; and it brings lubricating oil to the rings.
In addition, it has appeared that, although the pressure in the air jet is well below that which exists in the driving cylinder at the moment when the ports are uncovered, the air jet deflects the burning gas jet erupting from the driving cylinder away from the rings.
Each air delivery means associated with a driving piston cooperating with exhaust ports comprises at least one delivery passage or channel per exhaust port. All channels can be supplied with air from a same annular chamber, connected to a tank storing scavenging air for the engine.
It may be noted that the approach is not suitable for use in a Diesel engine where air available after pressurization in the engine crankcase is under a pressure too low for exhaust gas deflection. The invention requires availability of air under an adequate pressure and preferably slightly loaded with lubricant.
The cylinder wall is also subjected to considerable heat stresses, particularly in the head portion thereof, where it is continuously uncovered by the driving piston. That wall typically comprises a jacket force fit into an annular part bearing the fuel injection valve. The cylinder may preferably be provided with a cooling circuit comprising a circular channel provided between a ring of the cylinder and the jacket. That channel communicates longitudinal inlet channels for cooling liquid and return channels interleaved with the inlet channels.
Each driving cylinder is typically provided with two reciprocating pistons whose movements toward and away from each other are synchronised. Then the wall of the cylinder will generally include a central ring in which two jackets are fitted, one of which is provided with exhaust ports and the other with intake ports. The cooling circuit of the cylinder wall may then include axially directed inlet grooves for cooling fluid, formed along that jacket which is provided with exhaust ports; the circuit may further include the circular channel formed in the jacket, fluid return grooves and outlet holes communicating fluid from the return grooves to a space formed around the central ring. A similar channel and groove system is associated symmetrically with the jacket provided with intake ports.
The invention will be better understood from the following description of a particular embodiment thereof given by way of example.